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Thursday, May 26, 2011

I've been thinking about the idea of weird geology, ever since Dana at En Tequila Es Verdad introduced the topic for Accretionary Wedge #34 — I haven't been thinking about it long and hard, mind you, because I've been busy writing geo things at work, using up both my writing and thinking capabilitites. I wrote a little, thought a bit; then, after thinking some more, I thought I’d finish up this morning at the laundromat, but it tuns out they don’t have wi-fi, which is one thing I find not only entirely weird, but just a little bit irritating.

I have a few thoughts about geological weirdness, the biggest of which is that after all these many years (however long it’s been), I am no longer as surprised by the things rocks can do as when I was first starting to map as part of my job back in the late 1970’s. Back then, I learned an important lesson that has stuck with me over the years, which is: if the rocks are doing something you don’t think is possible or which you have never heard of or read about, look again. Have another geologist come check out your contacts and the nature of whatever weirdness you think you spotted, and brainstorm some ideas. Chances are, you have run across something merely unusual but not impossible. You will have to go with whatever the rocks are telling you that they did on their way to becoming whatever thing they are now.

Volcanic rocks, inside or outside the Huge Caldera settings sometimes known as "Supervolcanoes," can do some particularly unexpected things. When anywhere near them, learn to expect the unexpected and to throw out preconceived notions as soon as it becomes clear that you need to (which isn’t all the time, admittedly). Apply this way of thinking to other, non-volcanic settings. Over the years, expecting the unexpected will serve you well.

Other places to expect particular geologic weirdness are 1) anywhere in the Basin and Range province and 2) any time you are anywhere near a metamorphic core complex or where you suspect that you are in detachment or extreme extension terrane. I’ve found tilted Precambrian rocks above a 50-foot thick, crumbly, weakly mineralized mesobreccia of gneissic rocks, all sitting on top of tilted Tertiary sediments that included soft, punky lake beds. Possibly this unusual tectonic stratigraphy of very-much-older over very-much-younger was not produced by extreme extension, possibly the Precambrian was thrust over the middle to late Tertiary via a thrusting method related to left-lateral strike-slip faulting along the Garlock Fault. Who knows? We never drilled deep enough to find out what lay beneath the tilted Tertiary rocks beneath the tilted Precambrian rocks.

As for weird today, I'll just leave you with these pictures of a giant breccia (breccia in the foreground, breccia in the background). We were driving into the slot-canyon portion of Titus Canyon, a one-way dirt route into Death Valley from just south of Beatty. We drove around a curve in the road, suddenly coming face to face with a megabreccia. I slammed on the brakes (not that I was driving very fast to begin with). The same angular rock fragments from the first photo can be seen in this second photo, this time without anyone for scale. The breccia extends at least 35 to 40 feet up the wall.In reading up on this breccia, I found more than one explanation, including more than one kind of sedimentary origin to vaguely cited tectonic causes. Anyone have any ideas?

14 comments:

Hi Silver Fox. Great blog! I've been to Death Valley many times and love driving through Titus Canyon. My understanding on the polished megabreccia is that the limestone was fractured (stress source unknown because no conspicuous faults in the immediate area - probably regional) with subsequent limestone dissolution and calcite mineralization due to superheated fluid migration. Similar to what you read? Robert

Thanks, Robert. If you have access to or know of anything specific about the breccia, I'd love to have the references. Most things I've read just cite it as a fault breccia with no further comment, other things I'm not sure are referring to the same breccia, e.g. this about the Tertiary Titus Canyon Formation. Locations in the pdf are obscure, and no photos of the breccias referred to.

Additionally, a couple refs mention collapse breccia, as though the thing formed from cave collapse, but those citations are general and made by passers-through, not by geologists.

Anyone wanting to comment on this blog or other Blogger blogs may need to use Firefox rather than Internet Explorer. Blogger does not have the sign-in problem fixed. I just entered an endless loop trying to comment here, even though they recognize my sign in at the Blogger dashboard. Ugh.

That may be the most memorable outcrop of many in Titus Canyon. I've been on that drive I think 6 times, and another walk-in from the top of the fan. The megabreccia is a short walk from the parking lot at the mouth of the canyon, and if your time budget is limited, that may be a preferable option than the several hour drive. However, if you do have time, the whole drive through is awesome.

Regarding its origin, I know I've read suggestions in a couple of places that it might be a hydrothermal/phreatic breccia, but almost everything I've read that's actually coming from geologists includes that coded geospeak phrase, "not well understood." Which in regular English means "no one knows with any confidence."

Does the Oligocene Titus Canyon Formation include the breccia? It includes megabreccia. It has a carbonate breccia at the base, grading laterally or upward into a sedimentary breccia, and also into mudstones and conglomerates, all having something to do with a tectonic basin. Or something like that.

Mr. Lepeer's explanation is what I have heard also. I visited that formation while in a UCLA geology class, and heard that explanation, those it was admittedly uncertain. It also makes sense given the nature of that limestone, which is Cambrian. If you explore around, you will find many types of calcite formations in the limestone (unfortunately all my pictures of it are pre-digital). The calcite formations are not limited to this breccia formation.

Silver Fox. (this is dmbeaster, but I may sign in under another name) Was rereading this, and the Titus Canyon Formation and its breccia formations are different from this limestone exposure. The Titus Canyon Formation is located mostly northeast of your picture location by around 10 miles. They are Oligocene in age - this limestone is Cambrian. The Titus Canyon Formation is a sediment basin deposit, whereas the limestone here is marine.

I also find the hydrothermal explanation as the most plausible. Who knows when it occurred, but long after this limestone was deposited and buried. As I mentioned above, there are other calcite formations in the area found in this limestone which also suggests a hydrothermal source, although they are not breccia formations. I was looking for my pictures of these features taken long ago, but did not easily find them.

Thanks, David/dmbeaster, for the confirmation. I'd decided the Oligocene breccia couldn't be the same thing, though it is described as containing a megabreccia at the base. There may be some info on the Titus Canyon breccia in a Special Paper, but so far, I've been too busy to see if I can look that up through Bloc of Docs, or if I have to buy the whole thing. Also, still meaning to get the Death Valley geology and hikes books mentioned by Robert Leeper.

I particularly recommend the hiking book that he mentioned by Digonnet. I am a lifelong hiker and read these kinds of books for fun. That is an amazingly good book with great hike descriptions. Death Valley has many interesting narrows in the side canyons, and is a great place to go hiking from October through April.

I found it interesting that you posted these pictures of the exact same formation that I saw as part of a geology field class 35 years ago, and many times since.